1. Field of the Invention
The present invention relates to impedance matching of a digital output driver to a transmission line, and more particularly, to an adaptive output driver circuit that comprehends variations in circuit impedance due to manufacturing variation, power supply levels, and temperature, and adjusts its output impedance to match the transmission line impedance.
2. Description of Related Art
Digital electronic circuits often produce output signals that are communicated to other electronic circuits via a transmission line, and for that reason it is known to include an output driver for the purpose of generating the output signals having desired voltage and timing characteristics. In order to achieve high speed signaling, the impedance of the transmission line must be matched to either the source of the output signal and/or the destination (or load) in order to avoid reflections of the output signals that degrade the timeliness of the signal communication. Source impedance matching is achieved by matching the impedance of the output driver to the impedance of the transmission line. With source impedance matching, the output signal will pass from the output driver onto the transmission line without reflection. The signal will then propagate down the transmission line to the load. If there is an impedance mismatch at the load end, the signal will be reflected back down the transmission line to the source, and the reflected signal will be terminated at the source end by the impedance match.
There are several known methods for achieving source impedance matching. One such method is to add an external series resistor to a low impedance output driver so that their sum approximates the impedance of the transmission line. FIG. 1 illustrates an exemplary source impedance matched output driver communicating with a device under test through a transmission line. Specifically, an output driver 12 drives a capacitive load 34 of a device under test or DUT 32 through a transmission line 20. The output driver 12 has an impedance Zd, and the transmission line 20 has a characteristic impedance Zo. A series resistor 14 having a resistance Rs is coupled in series between the output driver 12 and the transmission line. The series resistor 14 is selected such that transmission line impedance Zo equals the sum of the output driver impedance Zd plus the resistance Rs.
A closed-loop amplifier may be used as the output driver due to its very low impedance so that most of the required source matching impedance appears in the added series resistor. A drawback of this approach is that low impedance output drivers tend to be physically large, requiring additional component and board area space as well as additional power. Alternatively, an open-loop output driver may be used, such as a simple digital buffer, which takes up less space and has lower power requirements. But, the impedance of the open-loop output driver is significant in comparison with the transmission line impedance, is non-linear, and (for MOS circuitry) varies greatly with processing and temperature changes. As a result, resistor selection for an open-loop output driver is often difficult.
An additional consideration is that it is desirable to include the series resistor within the same integrated circuit package as the output driver in order to minimize the additional external circuitry. This further exacerbates the difficulty in selecting an appropriate series resistor since it is difficult to produce a resistor on an integrated circuit that is stable over the processing and environmental conditions. In some cases, the resistance value may vary by as much as 50%, making it very difficult to accurately match the transmission line impedance. A trimmable thin-film resistor may be used to provide on-chip integration of the series resistor, or fusible links may be incorporated with a resistor array that enables selection during manufacture of an appropriate net series resistance. Both of these approaches tend to extend unit testing, and may be susceptible to aging and environmental variations. They can also raise additional quality assurance concerns above standard processing.
Another known approach to achieving source impedance matching is to designing the size of the transistors of the output driver to have the required resistance. In CMOS design, this is done by specifying the width of the output devices. This approach also has drawbacks in that the variation in typical manufacturing processes can cause the transistor output impedance to vary significantly. Moreover, the transistor output impedance further varies with power supply levels and temperature.
Signal inspection based control systems have also been proposed. One such control system examines the initial step amplitude when the output driver launches a signal transition into the transmission line. In a matched system, the initial step amplitude is ideally half of the total amplitude of the digital signal specification. Another such control system requires a round-trip external loop made of the transmission line, and matches the resulting signal at the end of the loop with the intended drive signal by integrating both signals. These control loops require constant signal transmission, and may require a dedicated, sample transmission line.
Yet another approach is to provide closed-loop, continuous-time impedance matching using feedback and high-gain op-amps for driving analog signals onto controlled-impedance cabling. These are complex designs requiring fast op-amps, careful AC stability analyses, and usually, external resistors for the feedback network because of the resistance accuracy required.
Accordingly, a need exists for an adaptive output driver circuit that detects variations in circuit impedance due to manufacturing variation, power supply levels, and temperature, and adjusts its output impedance to match the transmission line impedance, while overcoming the drawbacks of the known source impedance matching approaches.